The anthocyanins are glycosides of soluble coloring materials of plants. They are water-soluble pigments which are dissolved in the cell sap of plants. The various shades of red, blue, purple and violet of fruits and flowers are due to these pigments. The variations in color are due to slight alterations in the molecule which do not affect the fundamental chemical structure. The anthocyanins are amphoteric and form salts with both acids and bases. They usually occur as mixtures which vary from plant to plant. The color of the pigment is determined by the pH of the medium in which it is dissolved.
As natural products which occur in most fruits and leaves, the anthocyanins are desirable coloring agents for foods and drinks for human consumption. They have very low toxicity which gives them advantages over synthetic coloring agents for food products.
The conventional method of anthocyanin recovery from plant materials involves extraction with dilute alcohol solution of HCl, purification by ion-exchange and acid stabilization (Chiriboga and Franics, J. Am. Soc. Hort. Sci., 95(2):233-236, 1970). The mineral acid used for the elution and stabilization of anthocyanins limits the utilization of extracted anthocyanins due to the low pH imparted when added to food products. Ion-exchange purification is tedious and the cationic resins normally used to purify the anthocyanins also concentrate undesirable metal ions in the recovered anthocyanin. Weak cationic exchangers have low pigment capacity, and strong cationic exchangers, though excellent anthocyanin absorbers, require large volumes of solvent for complete elution of anthocyanins.
This invention has the advantage of avoiding mineral acids and ion-exchange materials which have previously been used in isolating anthocyanins from plants. This specification describes a new procedure for anthocyanin recovery from grape wastes based on tartaric acid-alkanol (methanol) extraction followed by precipitation of excess tartaric acid as potassium hydrogen tartrate. The centrifuge residue and wine grape pomace are wastes generated during the production of grape juice and wine, respectively. Their high anthocyanin and low sugar contents make it possible to recover the anthocyanins by the method described in this specification.